The biomass burning organic aerosol (BBOA) was quantified and characterized during a period of elevated emissions from residential heating at a Mediterranean urban area. Positive Matrix Factorization (PMF) of the measurements from a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) indicated that biomass burning organic aerosol (BBOA) was the dominant organic aerosol (OA) component during this period accounting for 52% of the total OA with the rest of the sources being traffic (10%), cooking (13%) and oxygenated OA (OOA 24%). One fourth (25%) of the BBOA had undergone chemical aging and was identified as aged BBOA. Organonitrates correlated highly with this aged BBOA factor. Reactions of the biomass burning emissions with nitrate radicals during nighttime appeared to be the major source of aged BBOA. Our findings strongly suggest that the BBOA estimated from AMS factor analyses in urban environments during the winter is a lower limit of the biomass burning contribution to OA. A significant fraction of the OA from biomass burning is included in the OOA factor. The average PM 2.5 oxidative potential (DTTm) measured during the campaign was 14 +/- 4.5 pmol min- 1 mu g-1 , elevated during periods when BBOA chemical aging was favorable.